This invention relates to a plastic-packaged semiconductor device which has a heat sink.
It is a recent trend that a plastic package is very often used for packaging a semiconductor chip by transfer molding a resin, such as epoxy resin, to form a semiconductor device. Such a semiconductor device may be called a plastic-packaged semiconductor device. The plastic-packaged semiconductor device inevitably includes a heat sink so as to irradiate heat which is caused to occur from the semiconductor chip. Otherwise, the semiconductor chip is degraded or deteriorated in a temperature characteristic.
Conventionally, a wide variety of heat sinks have been proposed and practically used in a plastic package for accommodating the semiconductor chip. However, most of proposals have been offered to make each thermal expansion coefficient of the heat sinks close to that of a semiconductor material, such as silicon, gallium arsenide, which forms the semiconductor chip. This might be in fear of breakage in the semiconductor chip due to a difference of the thermal expansion coefficients between the semiconductor chip and the heat sink. Taking the above into consideration, a high refractory metal, such as molybdenum, tungsten, or the like, has been used as a material of the heat sink because the high refractory metal is close to the semiconductor material in thermal expansion coefficient.
Meanwhile, it is to be noted that recent requirements are to make a semiconductor device, a heat sink, and the like, light in weight and to cheaply manufacture such a semiconductor device in a mass production fashion. In order to satisfy such requirements, use of the high refractory metal is unsuitable because the high refractory metal has a high density, which makes the heat sink heavy in weight.
According to the inventors' experimental studies, it has been empirically found out that no breakage takes place in the plastic-packaged semiconductor device even when each of the heat sink and the plastic material has a thermal expansion coefficient greater than that of the semiconductor material to some extent. This shows that the thermal expansion coefficient of the heat sink used for the plastic-packaged semiconductor device may not be always close to that of the semiconductor material of the semiconductor chip. In this event, it is preferable that the thermal expansion coefficient of the heat sink does not exceed the thermal expansion coefficient of the plastic material. From this fact, it is readily understood that the heat sink may be formed by a material which is lower in density than the high refractory metal and which is light in weight.
In addition, various kinds of heat sinks are preferably prepared for a wide variety of the plastic-packaged semiconductor devices. As a result, it is desirable that a material of such a heat sink can be readily shaped into various configurations.
Practically, consideration might be made about forming such a heat sink by copper which has a thermal expansion coefficient of 17.times.10.sup.-6 /K lower than the thermal expansion coefficient of 18.times.10.sup.-6 /K in epoxy resin. However, the heat sink of copper is undesirably deformed with an increase of a temperature of the semiconductor chip and weak in mechanical strength.
In general, heat sink materials have been proposed in Japanese Patent Applications Nos. 288,517/1991, 20,725/1992, and 3,056/1992 by the instant inventors and are suitable for ceramic packages which mount or package semiconductor chips.
Specifically, a mixture of copper and molybdenum is sintered and pressed in Japanese Patent Application No. 288,517/1991 to form a heat sink sheet composed of the mixture. However, no consideration is made at all in Japanese Patent Application No. 288,517/1991 about application of the heat sink sheet to a plastic-packaged semiconductor device which has a thermal expansion coefficient greater than that of the ceramic package. Accordingly, it is difficult to know whether or not the mixture mentioned in the above-referenced application is suitable for the plastic-packaged semiconductor device.
Alternatively, a composite material or grain consists of molybdenum particles enveloped by copper films in Japanese Patent Application No. 20,725/1992 while a composite material consists of tungsten particles enveloped by copper films in Japanese Patent Application No. 3,056/1992. Such composite materials may be applied to heat sinks used for the plastic-packaged semiconductor devices. However, it has been found out that the composite materials are not suitable for manufacturing the plastic-packaged semiconductor devices in a mass-production fashion because a total amount of copper can not be kept at a predetermined range.
Anyway, disclosure is made neither about a plastic-packaged semiconductor device nor about a heat sink suitable for the plastic-packaged semiconductor device in the above-mentioned applications. Therefore, it is difficult in the prior patent applications to consider demands for the plastic-packaged semiconductor devices.